Orbital Topology Overcomes Energetic Constraints: Conjugation Expansion Breaks the Inverse Relationship between SOMO Splitting and Spin Coupling for Giant Ferromagnetic Coupling in Organic Diradicals

The commonly assumed inverse relationship between magnetic exchange coupling ( J ) and the energy gap between two singly occupied molecular orbitals (Δ E SS ) has long guided the design of organic diradicals. Contrary to this well-established doctrine, we demonstrate an unprecedented violation: the concurrent increase in both Δ E SS and J through conjugation expansion engineering in the PAH-bridged diradicals, unveiling an “inverted SOMO splitting effect”. Across a controlled PAH size series, J surges from 585 cm –1 to over 2147 cm –1 (nearly a 3.7-fold increase) even as Δ E SS widens from 0.10 to 1.15 eV. We propose a novel mechanism of synergistic orbital separation and spin delocalization. Extended conjugation induces orbital mixing between two symmetry-adapted radical-group orbitals and HOMO or HOMO–1 of the coupler, yielding a low-lying local SOMO between two radical groups and a high-lying SOMO with a delocalized distribution on the coupler as spin-transport network-channels. This cooperative action of symmetry-governed orbital separation and spin network effectively overcomes the energy penalty associated with enlarged Δ E SS, resulting in a remarkable enhancement of J . Substituent perturbation analysis further shows that J becomes progressively less sensitive to local electronic effects as conjugation increases, supporting a transition from an energetic to orbital-topological control. These results establish orbital spatial topology engineering as a practical molecular design strategy for achieving simultaneously large SOMO splitting and strong magnetic coupling in organic diradicals.